The position of the Arctic treeline is an important regulator of land surface energy budgets, ecosystem-atmosphere carbon cycling, wildlife habitat and availability of subsistence resources to local communities.  The prevailing hypothesis states that treeline position is determined by air temperature during the growing season.  Because trees are taller than tundra vegetation and their canopy extends above the warmer boundary layer, their tissues are colder than tundra vegetation.  These colder conditions are hypothesized to limit cell division and growth, such that seedlings are unable to grow into trees.  However, our early work revealed that air temperature is warmer than previously thought near the Arctic treeline in Alaska and the indirect effects of temperature on soil nutrient availability may be more important determinants of tree growth.  We hypothesized that cold soils at treeline, particularly during winter, limit microbial activity and nutrient availability to the point where trees are barely able to survive and grow.  In this project, we used snowfences over five winters to experimentally increase snow depth around eight trees at each of three treelines that varied in soil moisture and tundra vegetation near the Agashashok River in northwest Alaska.  Trees that were treated with snowfences experienced warmer soils in winter and no evidence of a change in growing season soil moisture, when compared with control trees.  The snowfence trees at our wet and moist treeline sites tended to have higher concentrations of nitrogen in their needles and significantly greater branch primary growth than control trees.  This finding corroborates observations made in nearby long-term study plots where branch primary growth is significantly positively correlated with the depth of the previous year?s late winter snowpack.  The snowfences also led to a decrease in fine root growth and the size of the fine root standing crop, suggesting that trees at our wet and moist treeline sites may have shifted allocation of their resources from roots to shoots.  There was no evidence that experimental deepening of the winter snowpack affected needle nutrient concentrations or growth of trees at our dry treeline site.  There is also no evidence thus far that the snowfences have affected white spruce cone production, main stem radial growth or overwinter mortality of fine roots.  Overall, our results contradict the prevailing hypothesis of treeline formation and suggest that deeper winter snowpacks will improve the nutrition of treeline white spruce, decrease their investment in fine roots and increase development of their canopy.

In addition to supporting our treeline snowfence experiment, this project supported valuable collaborative opportunities with other researchers on related topics.  We worked with a Fulbright Fellow on a study of long-term vegetation change and with the United States Geological Survey on a study of the sensitivity of headwater streamflow to vegetation change and thawing permafrost, both within our Agashashok River study area.  We also contributed data and intellectual resources to a synthesis of winter carbon dioxide emissions across the global northern permafrost region.  Finally, measurements of winter carbon dioxide emissions from our snowfence study plots showed evidence of lower microbial activity at a given soil temperature near the end of warm winters with deep snowpacks than at the end of cold winters with shallow snowpacks.  We hypothesized that microbes might deplete the pool of available labile carbon during winters with warm soils.  We tested our hypothesis by experimentally adding labile carbon to soils in both the field and under controlled temperature conditions in the laboratory.  Our results confirmed that the soil microbes can exhaust the supply of labile carbon available to fuel respiration over the long winter season, leading to lower-than-expected carbon dioxide emissions and changes in soil nutrient cycling.  

Last Modified: 01/16/2023
Modified by: Patrick F Sullivan